Recently, superficially porous particles have drawn great interest because of their special characteristics and improvement in separation efficiency. Superficially porous metal oxides, particularly silica particles, are used in chromatography columns to separate mixed substances from one another. Such particles have a nonporous, solid core with an outer porous shell. High-pressure liquid chromatography (HPLC) columns using superficially porous silica particles allow shorter mass transfer distances, resulting in faster mass transfer and separation than conventional columns.
Various methods to prepare superficially porous silica particles have been reported. (U.S. Patent Publ. No. 2007/0189944 A1.) The first method is a spray-drying approach where solid silica particles or cores are mixed with a silica sol. The mixture is sprayed under high pressure through a nozzle into a drying tower at high temperature (e.g., 200° C.). Unfortunately, the particles prepared this way often are incompletely or un-homogeneously coated. Such particles invariably also contain significant concentrations of unwanted totally porous particles of similar size, which come from the sol. Elutriation-fractionation of this product often fails to remove the totally porous particles, making the spray-drying approach less than optimal for producing the desired superficially porous particles. In addition, the spray-drying method can only make particle sizes larger than 5 μm, mostly in the 30 μm to 100 μm range, and such particles tend to have broad particle size distributions.
A second conventional method is a multilayer approach, in which solid (i.e., non-porous) silica cores are repeatedly coated with layers of colloidal particles by alternating layers of oppositely charged nanoparticles and polymers containing amino-functional groups until the particles reach the desired sizes. (U.S. Pat. No. 3,505,785; U.S. Patent Publ. No. 2007/0189944 A1.) The process is labor intensive and very difficult to practice. When the method is applied on small cores with size less than 2 μm, the final particle surface tends to become less spherical and rougher. The process generates many different types of aggregated particles, resulting in loss of yield of the desired particles.
A third conventional method involves coacervation. In this method, solid (i.e., non-porous) silica spheres are suspended in a coacervation reaction mixture including urea, formaldehyde, and colloidal silica sol under acidic conditions. A coacervate of urea-formaldehyde polymer and ultra-pure silica sol is thus formed and becomes coated on the solid spheres (Kirkland 2000 J. Chromatography A 890, 3-13). The urea-formaldehyde polymer is then removed by burning at 540° C. The particles are then strengthened by sintering at an elevated temperature. This procedure is much simpler and more practical compared to the multilayer approach described above. However, the coacervation method has its drawbacks. One is that some of the solid particles often are not coated, leaving non-porous particles in the finished product. Another drawback is that much smaller totally porous particles are formed along with the coated and uncoated particles, necessitating further classification of totally porous particles and superficially porous particles.
Thus, conventional methods of preparing superficially porous silica particles all use solid silica nanoparticles as the building blocks on which an outer porous shell is added. As a result, the porous shell has randomly distributed pores with wide pore size distribution. The pore size is mainly controlled by the size of silica nanoparticles and the tortuous pore channel is determined by how those nanoparticles randomly aggregated. These superficially porous particles with a solid core and a porous layer usually have a particle density between 1.5 cm3/g to 1.7 cm3/g, which is much higher than 0.8 cm3/g to 1.2 cm3/g for the totally porous particles. The high particle density mainly contributed from the solid core that possesses 70 vol.% to 80 vol.% of the particle. Moreover, the resulting rough external particle surfaces limit the performance of columns using such particles at high flow rates due to an unusually high film mass transfer resistance. Rough surfaces also limit the packing density because of increased friction forces among particles during the packing process. (Gritti, et al. 2007 J. Chromatogr. A 1166, 30-46).
When used as solid phase in separation columns, the chromatographic performance is highly related to how well the column is packed. An efficient packing technique is to use a slurry solvent whose density approached that of totally porous silica particles. So the particles tended not to settle during the column packing process. Also higher packing pressure generally was found favored for both performance and stability. (Kirkland 2006 J. of Chromatography A. 1126, 50.) As a result it is much more difficult to pack superficially porous particles because of their high particle density. Not only it is hard to find a high density solvent to match particle's density but also require strong particles to withstand the higher packing pressures.
Thus, there is a need to make both superficially porous silica particles with a narrow particle size distribution, narrow pore size distribution, high specific surface area and a porous outer layer for faster separation, lower chromatography column pressure drop, and higher efficiency, together with stability at high pH and with good mechanical strength under chromatography conditions.